Hydrogen and helium trapping in hcp beryllium

Even though hydrogen-metal surface interactions play an important role in energy technologies and metal corrosion, a thorough understanding of these interactions at the nanoscale remains elusive due to obstructive detection limits in instrumentation and the volatility of pure hydrogen. In the present paper we use analytical spectroscopy in TEM to show that hydrogen adsorbs directly at the (0001) surfaces of hexagonal helium bubbles within neutron irradiated beryllium. In addition to hydrogen, we also found Al, Si and Mg at the beryllium-bubble interfaces. The strong attraction of these elements to (0001) surfaces is underlined with ab-initio calculations. In situ TEM heating experiments reveal that hydrogen can desorb from the bubble walls at T ≥ 400 °C if the helium content is reduced by opening the bubbles. Based on our results we suggest the formation of a complex hydride consisting of up to five elements with a remarkably high decomposition temperature. These results therefore promise novel insights into metal-hydrogen interaction behavior and are invaluable for the safety of future fusion power plants.


Bubbles at elevated temperatures
If bubbles are not opened during the heating procedure, both helium and hydrogen intensities do not change significantly at elevated temperatures.
Supplementary Figure 3: He and H in a closed gas bubble at room temperature (RT) and at 750 °C. [1] The red crosses mark the position for the spectra shown in A 4.
Supplementary Figure 4: EELS signal from the bubble surface of the bubble shown in A 3 (see red crosses) at room temperature (RT) and 750 °C. The increased temperature leads to a decrease in the valance electron density which is visible as a Be bulk plasmon shift to the left. [4] Helium density, pressure determination and helium signal A common procedure to estimate the He density in closed gas bubbles is the one presented by Walsh et al. [5] The He density nHe is given by where IHe and IZLP are the integrated intensities of the He K-edge and the zeroloss-peak, respectively d is the bubble thickness at the pixel position and σHe the cross-section of the He 1s2p transition, which was calculated using Sigmak3 [4]. The error of this density calculation is about ± 30 % and is mainly caused by the thickness measurements. With an acceleration voltage of 300 kV and a semi collection semi-angle of 18.8 mrad our experimental setup leads to a cross-section of the He 1s2p transition of 7.747 × 10 -19 cm 2 for a 4 eV integration window.
Supplementary Figure 5: Helium density inside a closed gas bubble in Be using equation (1).
The pressure inside the bubble can be estimated using the semi-empirical equation of states (EOS) proposed by Trinkaus [6]. The pressure P inside a bubble is given as a function of the helium density n in atoms/angstroms and the temperature T in Kelvin.
The coefficient is given by For the bubble in A 5, this results in a pressure of 4.4 × 10 -2 GPa at 293 K.
Supplementary Figure 6 shows the helium blue-shift due to the high helium density in the bubble interior.
Supplementary Figure 6: Helium peak shift during bubble opening. The red curve shows the signal of a closed bubble. The blue signal was acquired just after the bubble was opened and the pressure dropped consequently. [1] Furthermore -for the first time -we measured a high-energy-resolution EEL spectrum of helium that shows four different electron transitions (see Figure 1 (c)) using STEM EELS. In the past only the 1s2p and 1s3p transitions have been measured using EELS [7][8][9][10].
More transitions were only visible in specially designed electron scattering experiments [11].

Hydrogen peak
Theoretically the observed feature at around 12-13 eV at the bubble walls could be attributed to the Be-surface plasmon. According to the Drude Model the position of the surface plasmon Es in the spectrum is given by where Ep is the bulk plasmon energy. The surface plasmon would appear in the range of Ep/√3/2 for l=1 and Ep/√2 for l∞ which gives 13.2-15.3 eV. However, in our opinion several things speak against the surface plasmon and for hydrogen: (i) Not all closed bubbles, even if they had the same sizes, showed the 12-13 eV peak. (ii) The 12-13 eV peak showed similar intensities for variously sized bubbles. (iii) Bubbles that have been cut open already during FIB preparation of the lamellae do not show the 12-13 eV peak. It is known, that Beryllium oxides very quickly, however the formation of thin oxide-layers does not necessarily prevent the occurrence of a surface plasmon. [12] (iv) The 12-13 eV peak is also present in the bubble interior, albeit with a weaker intensity. The strong delocalisation of surface plasmons was only reported for energies > 3 eV in the past. Surface plasmons with energies < 10 eV undergo only a shift of several nanometers. [12] (v) The observed peak at 12-13 eV was located constantly at the same energy within the individual bubbles and was generally slightly below the expected energy range for the beryllium surface plasmon.

(vi)
For us there is no reason why the plasmon peak intensity should decrease with increasing temperature as strong as we observed it. Although there have been reports about a red shift and broadening of the surface plasmon resonance in Au nanoparticles as the result of an increased temperature [13], they do not disappear completely as it is the case in our experimental investigations. (vii) Recent Atom Probe Tomography examinations at the Culham Science Center for Fusion Energy (CCFE) [14] detected appearance of the tritium related massspectrum peaks in the same neutron irradiated beryllium samples used in this work. The corresponding detected atoms appeared in disk-like shapes with dimensions similar to the helium-tritium bubbles observed in the present EELS work. The APT data is still under evaluation and will be soon published in a separate research paper. (viii) Temperature Programmed Desorption (TPD) experiments [15][16][17][18] revealed simultaneous "burst" release of helium and hydrogen from bulk samples when irradiated Be pebbles are heated to temperatures ≤ 1100 °C. This observation indicates that both elements, helium and hydrogen, must be present within bubbles. (ix) DFT simulations revealed that hydrogen atoms which adsorb on Be (0001) surfaces may lead to a substantial reconstruction of the beryllium surface [19,20]. Since hydrogen atoms prefer a two-fold coordinated bridge position BeH2 polymer chains are formed. Polymer-like structures, however, will have a different electronic structure compared to the bulk material and hence also a deviating